GlobalOpt: Preserve comdats of unoptimized initializers

Rather than slurping in and splatting out the whole ctor list, preserve
the existing array entries without trying to understand them.  Only
remove the entries that we know we can optimize away.  This way we don't
need to wire through priority and comdats or anything else we might add.

Fixes a linker issue where the .init_array or .ctors entry would point
to discarded initialization code if the comdat group from the TU with
the faulty global_ctors entry was dropped.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218337 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Teach the new vector shuffle lowering to lower v4i64 vector
shuffles using the AVX2 instructions. This is the first step of cutting
in real AVX2 support.

Note that I have spotted at least one bug in the test cases already, but
I suspect it was already present and just is getting surfaced. Will
investigate next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218338 91177308-0d34-0410-b5e6-96231b3b80d8